Obesity is a major risk factor for chronic disease. Besides our obesogenic lifestyle, people?s innate physiology/metabolism, encoded by their genomes, is another major factor that determines why some people gain weight more easily than others. Genetic studies in animal models and humans, have shown that the brain is a key player in body weight regulation. In particular genes that constitute the leptin-melanocortin pathway have been repeatedly implicated in obesity; i.e. mutations in LEP, LEPR, POMC, MC4R, PCSK1 and SH2B1 have been frequently reported to cause early-onset severe obesity, likely through influencing appetite, hunger, satiety, reward, etc. In addition, common variants near the same genes have been identified in GWAS for obesity/BMI. In a recent, large-scale exome-wide study, we showed that a nonsense mutation (Tyr35Ter) in MC4R was associated with a ~15lbs higher body weight in carriers. MC4R Tyr35Ter was identified >20 years ago as a high- impact monogenic mutation that causes severe early-onset obesity. In-depth functional analyses showed that 35Ter results in a complete loss of function of MC4R. Yet, of the 66 mutation carriers in the UK Biobank (N~500,000), nine (14%) were of normal weight and had never been overweight, beating their genetic odds. This observation sparked our interest in investigating all mutations in genes of the leptin-melanocortin pathway, reported to cause obesity. By in-depth comparison of normal weight carriers with obese carriers, we aim the reveal compensatory mechanisms (genetic, non-genetic) that prevent weight gain, not yet targeted for prevention and treatment. However, mutations reported so far as the ?cause? of obesity have been identified in small-scale case-biased populations, and their penetrance and effect on obesity at a population-level has never been estimated. As mutations are rare, determining their impact with rigor requires extremely large populations. With access to sequencing data from >220,000 individuals and genotype data from ~500,000 individuals (Mount Sinai BioMe Biobank, TOPMed Program, UK Biobank), we are uniquely positioned to (Aim 1) determine the impact of mutations in established monogenic obesity genes (LEP, LEPR, POMC, MC4R, PCSK1, SH2B1) through estimating their penetrance and assess their effect on obesity. Aim 2: To determine the characteristics that protect normal weight carriers of high-impact mutations from gaining weight, using phenotype/genotype data already available in BioMe, TOPMed, and UK Biobank. Aim 3A: To gain deeper insight in the underlying mechanisms by performing extremely deep-phenotyping in 60 participants recalled from the BioMe Biobank to assess energy intake, energy expenditure, lifestyle and other behaviors. Aim 3B: To investigate participants? iPSC-derived hypothalamic neurons for signaling characteristics and allele-specific expression. While traditionally, obesity research focuses on obese individuals and on the mechanisms that cause weight gain, we propose a paradigm shift. By studying normal weight mutation carriers and the mechanisms that protect them from gaining weight, we aim to reveal new targets for treatment and prevention in the general population.